The present invention concerns a container where alkaline solution is packed.
More specifically, it concerns a battery exterior container of alkali manganese batteries and the nickel cadmium batteries. Moreover, it concerns the surface treated steel sheet that is suitable for making thereof.
A method of barrel plating after press forming cold-rolled steel strip into a battery container or a method of press forming nickel-plated steel strip into a battery container have been adopted for manufacturing a battery container for such as alkali-manganese batteries and nickel cadmium batteries in which strong alkaline solution is packed. The reason why nickel plating is used for batteries such as alkali manganese batteries and nickel cadmium batteries is as follows. Nickel having excellent corrosion resistance to alkali is suitable for these batteries whose electrolyte is chiefly strong alkaline potassium hydroxide. Moreover, nickel is suitable for batteries since nickel has steady contact resistance when the battery is connected with an external terminal. Furthermore, nickel has excellent spot welding properties in case where each component is spot welded when assembled into a battery in the battery manufacturing process.
By the way, recently the main current of the plating method has been pre-plating, in which a steel strip is nickeled beforehand, replacing past barrel plating. It was difficult to manufacture products of high quality steadily by the past barrel plating because the difference of the plating thickness was large and it was especially difficult to nickel inside the container uniformly. As for pre-plating, the method of giving thermal diffusion processing after nickel plating has come to be applied chiefly to improve corrosion resistance. Now, the relation between the battery performance of the alkali manganese battery and the positive electrode container (battery container) is described hereafter. The said battery performance and the properties of the inside of positive electrode container are closely related. It is said that the lower the contact resistance between the inside of the positive electrode container and the positive electrode mix of the alkali manganese battery (consisting of manganese dioxide as a positive electrode active material, graphite as a conductor, and potassium hydroxide as electrolyte), the more excellent the battery performance. As for alkali manganese battery, the positive electrode mix and the positive electrode container are in contact, and the positive electrode container serves as not only a container of the battery but also as a conductor which transfers electrons.
Therefore, when the contact resistance between the positive electrode mix and the inner surface of the positive electrode container is great, the internal resistance of the battery rises. It causes decrease in the operation voltage, and in the electrical discharge duration, obstructing the battery performance. So, it is desirable to decrease the contact resistance between the positive electrode mix and the inner surface of the positive electrode container. Therefore, roughing the surf ace inner surf ace the positive electrode container, making a ditch on the positive electrode container in the vertical direction, and applying conductive coating or a conducting material made by adding binder to graphite are proposed to decrease the contact resistance between the positive electrode mix and the inner surf ace of the positive electrode container.
Next, the press-forming method of the battery container is described. Recently, DI (drawing and ironing) forming method is increasingly used as a method of thinning wall to increase the capacity of the battery replacing the past multi step deep drawing method (see Published Japanese Patent Hei 7-99686). This DI forming method and DTR (drawing thin and redraw) forming method is capable of increasing the battery capacity because the container side wall being thinner than the bottom thickness allows more positive electrode and negative electrode active materials to be contained. Moreover, the thick bottom has an advantage to improve the pressure resistance of the battery.
By the way, although the DI forming method and the DTR forming method are effective for increasing battery capacity as mentioned above, there is a disadvantage when they are used for continuous forming because the deformation resistance of the material in those methods is greater than in the conventional multi step deep-drawing forming method. Concretely, when the powdering quality (powdery dropout of the plating layer) in the cupping process of the DI forming method and the DTR forming method is inferior, the powder adheres to the die and the punch in the ironing process causing a defect in the container sidewall. Although the similar phenomenon happens in the deep drawing forming, the above-mentioned defect is more remarkable in the DI forming method and the DTR forming method because the container wall has small surface roughness, which produced more lustrous appearance.
Moreover, powdering quality is more critical in the DI forming method and the DTR forming method. Also, because the contact pressure of the material and the tool is greater in the DI forming method and the DTR forming method than that in the drawing method, favorable lubrication is requested for tool life. Therefore, materials which have a favorable powdering quality and retention of the press lubricant are requested.
When using nickel plated steel sheet, one of the ways to improve the retention of the lubricant is to cause cracks in the plating layer at the press-forming step and to hold the lubricant in the part where cracks are caused. As a means for this, gloss nickel-plating producing a hard plating layer is generally brought to mind. However, although the gloss nickel plating produces a hard gloss plating layer, it is brittle and it has inferior powdering quality at the press-forming. In addition, since gloss plating involves organic additives containing sulfur (for example, sulfonic acid having xe2x95x90C SO2-group) to make electrolytic ally deposited crystal grains fine, sulfur is absorbed in the plating layer, which causes the embrittlement with sulfur promoted by the temperature rise of the material in the ironing and the stretching process of the DI forming and the DTR forming resulting in a deteriorated powdering quality.
The inventors of the present invention examined various materials for a battery container having excellent formability in the DI forming method and the DTR forming method from such viewpoints, and found that nickel-cobalt alloy plating is suitable.
The present invention is based on such findings and it is aimed at a battery container having high quality and excellent continuous formability and a surf ace treated steel sheet which is suitable for producing the said battery container. Another objective of the present invention is to improve the removability DTR forming. This is taken into consideration since the difficulty of stripping the container from the punch (strippability) in the final pressing process is critical in the container manufacturing in addition to the above-mentioned powdering quality. At stripping where the container is pulled out from the punch hitching fingernails on the edge of the container, there was a problem that an inferior stripping caused breaking and split at the open edge portion of the container more frequently, which deteriorated the productivity.
A battery container which achieves the above mentioned purpose is obtained by forming a surface treated steel sheet, of which inside and outside are plated with nickel-cobalt alloy, using DI forming method or DTR forming method. The cobalt content of the above-mentioned nickel-cobalt alloy plating is preferably from 0.5 to 10 percent by weight, and the thickness of the above-mentioned nickel cobalt-alloy plating is from 0.5 to 3 xcexcm for the inside of the container and from 1.0 to 4 9 xcexcm for the outside of the container.
Another battery container is obtained by forming a surface treated steel sheet, of which inside and outside are plated with nickel-iron alloy, using DI forming method or DTR forming method. The iron content of the above-mentioned nickel-iron alloy plating is preferably from 0.5 to 5 percent by weight, the thickness of the above-mentioned nickel-iron alloy plating is from 0.5 to 3 xcexcm for the inside of the container and from 1.0 to 4 xcexcm for the outside of the container.
A first surface treated steel sheet is made by plating inside and outside a steel sheet with nickel-cobalt alloy. The cobalt content of the above-mentioned nickel-cobalt alloy plating is from 0.5 to 10 percent by weight.
Another surface treated steel sheet is made by plating inside and outside a steel sheet with nickel-iron alloy. The iron content of the above-mentioned nickel-iron alloy plating is from 0.5 to 5 percent by weight.
First of all, the formation of the nickel-cobalt alloy plating of the above-mentioned battery container and the surface treated steel sheet is described. When using a plating bath in which cobalt sulfate is added to Watts bath or sulfamate bath, eutectoid of cobalt and nickel is formed. As a result, with the increase of cobalt content in the plating layer, the hardness of the eutectoid plating layer increases. Concretely, the surface hardness of the plating by the nickel sulfamate bath rises to about 300 to 320 (Vickers hardness) when cobalt is added by 1 g/l (cobalt content 5%), while it is about 220 to 230 (Vickers hardness) when cobalt is not added.
Similarly, the hardness of the plating layer of the nickel-iron alloy plating reaches about 500 (Vickers hardness) when containing iron by 3 to 5% in the nickel plating layer. Thus, the surface treated steel sheet having hardened plating layer was manufactured, and then this surface treated steel sheet was formed into a battery container (alkali manganese battery LR6 type) using the DI forming method and the DTR forming method, etc. And when the sidewall inside and outside of the battery container was observed with a microscope, a fine surface roughening was observed.
In order to examine the powdering quality, the lubricant inside and outside of the manufactured battery container was removed with an organic solvent and then the powder dropped out from the plating layer was adhered to the adhesive tape and the amount was observed with the magnifying glass (magnification 25 times). As a result, remarkable decrease in the powdering quality was observed.
In order to evaluate the continuous formability of the battery container, powdering qualities in three kinds of forming methods, which are the deep-drawing method, the DI forming method, and the DTR forming method, were examined. It was found that nickel-cobalt alloy coated steel sheet and nickel-iron alloy coated steel sheet which constitute the surface treated steel sheet used in the present invention have smaller-punching load compared to the ordinary gloss nickel plated steel sheet.
When the surface treated steel sheet of the present invention is formed, the punching load is small because the lubricant enters the above-mentioned finely rugged surface of the surface treated steel sheet in the cupping process and the friction resistance decreases in the subsequent ironing process of the DI forming and the stretching process of the DTR forming due to the favorable retention of the lubricant between the materials and the die or the punch. It is greatly advantageous that the defects of the die and the punch due to metal-to-metal contact decrease because of the lowered punching load, resulting in longer die life and improved continuous productivity of the battery container. And favorable retention of the lubricant is also advantageous for the removability (strippability) of the battery container which is an important factor in the DI formability and the DTR formability.
The present invention is not only applied in the DI forming method and the DTR forming method as a means to thin the wall of the battery container, but also favorably applicable in the conventional multi-step deep-drawing method because it improves the powdering quality and decreases defects.
Moreover, when a battery is produced by forming the plated steel sheet of the present invention into an alkali manganese battery container using the DI forming method or the DTR forming method, the adhesion of the positive electrode mix to the inside of the container, and the adhesion of the graphite coating coated inside the battery container to the positive electrode mix after formed into the battery container are improved by the existence of the above-mentioned finely rugged surf ace.
That is, the use of the plated steel sheet of the present invention compensates the drawbacks that the surf ace roughness inside the container formed using the DI forming method and the DTR forming method is small and that the adhesion of it to the positive electrode mix or the graphite coating is inferior. Since the nickel-based alloy plating containing cobalt and iron of the present invention are involved in the iron-group which has excellent resistance to alkali corrosion and dissolution, it is a suitable material for the batteries such as the alkali manganese batteries, the nickel cadmium batteries, and the nickel hydrogen batteries of which electrolyte is high concentration alkaline solution.
By the way, the suitable cobalt content of the nickel-cobalt alloy plating is from 0.5% to 10%. When the cobalt content is less than 0.5%, it is not effective for hardening the cobalt plating layer. On the other hand, when the cobalt content is exceeding 10%, it is uneconomical because the hardening effect on the surface treated steel sheet is saturated and also because cobalt is expensive precious metal.
As for the nickel-iron alloy plating, the suitable iron content is from 0.5% to 5%. When the iron content is less than 0.5%, hardening effect cannot be obtained. On the other hand, when the iron content is exceeding 5%, the hardening effect is saturated, and it also causes difficulty to control the bath when the iron content increases any further. In relation to the plating thickness of the surface treated steel sheet of the present invention, the preferable range for the inside of the container is from 0.5 to 3.0 xcexcm both for the nickel-cobalt alloy plating and the nickel-iron alloy plating. On the other hand, the preferable range for the outside of it is from 1.0 to 4.0 xcexcm.
When the plating thickness of inside the container is less than 0.5 xcexcm, the exposure of the steel substrate is increased and the corrosion resistance is inferior in the container of batteries such as alkali manganese battery, causing deterioration in the battery performance due to dissolving out of the ferrous ion into the electrolyte. On the other hand, when the plating thickness outside the container is less than 1.0 xcexcm, rust might be generated on the battery container during press forming process and the battery manufacturing process and while preserving it for a long period of time since it does not have enough corrosion resistance.
The upper limits of the plating thicknesses inside and outside of the container are 0.3 xcexcm and 0.4 xcexcm, respectively, because the effect is saturated when the plating thickness exceeds these values and also it is uneconomical to make them any thicker.
Usually a low carbon aluminum killed steel is suitably used a base material of a surface treated steel sheet.
In addition, cold-rolled steel strip prepared from non-aging hyper low carbon steel (carbon content is less than 0.01%) with niobium and/or titanium added is also used. And the steel strip subjected electro-cleaning, annealing, and temper rolling after cold-rolling using a conventional method is used as a substrate for plating. After that a surface treated steel sheet is manufactured by plating the steel substrate with nickel-cobalt alloy or nickel-iron alloy.
As for the plating bath, however both of the known sulphate bath and the sulfamate bath can be used, sulfamate bath is suitable since it is comparatively easy to be controlled. Since either ratio of cobalt or iron to nickel deposited into plating layer is several times greater than that of concentration in either plating bath, it is possible to use nickel anode for anode and to supply cobalt and ferrous ions in the form of sulfamate or sulfate.